1. Field of the Invention
The present invention generally relates to an apparatus and a method for measuring a characteristic and a chip temperature of a light emitting diode (LED).
2. Description of Related Art
Recently, benefited from the drastic development of the epitaxy technology, LED semiconductor technology has successfully improved the light emitting efficiency of LED chips. As such, LEDs are more widely used, for example in projection pens, and illuminations. LEDs are known as having advantages of small sizes, longer lifetime, no contamination, and low cost. In addition, LEDs have better optical characteristics such as good color saturation, and ideal dynamic color control. In these manners, LED and related technologies are specifically concerned for further development.
However, when a high power illumination is demanded, for example, a larger current density has to be inputted. Unfortunately, under the situation that the light emitting efficiencies of the current LED chips is limited, most of the inputted power is converted into heat causing a drastic increase on heating generating density. In such a way, the overheat problem is a bottleneck in technology development. Thermal resistance, is defined as a quantity of a junction temperature of an LED being subtracted the ambient temperature and further divided by a corresponding input power. The thermal resistance is a criterion for evaluating a capability of heat dissipation of an LED package. A larger thermal resistance indicates a worse capability of heat dissipation, while a smaller thermal resistance indicates a better capability of heat dissipation.
When packaging an LED, an upper limit of the thermal resistance should be restricted. As such, the measurement of thermal resistances of components is important and representative. As to the parameters for calculating the thermal resistance, the ambient temperature can be conveniently obtained by measuring with a thermal couple. As to the thermal-generating component, the inputted power is also known and easy to obtain. However, a junction of a packaged LED is encapsulated inside, and therefore the junction temperature of LED chip is difficult to measure with a direct measurement. As such, the junction temperature is usually measured with an indirect electrical measurement. If the method can fast and accurate measuring the thermal conductivity and the effect on optical characteristics due to heat of the component to be tested, it would be helpful in facilitating to the design and testing of the thermal conductivity of LED chips.
Typically, conventional measurement mechanisms for LED chips are similar with those for ordinary IC chips. FIG. 1 is a schematic diagram illustrating a testing circuit for measuring a thermal conductivity of a conventional IC chip. Referring to FIG. 1, it is a real die method. There is no standardized thermal conductivity testing chip can be used for such a high power LED chip 100, and therefore the real die method is then adapted for the high power LED chip 100. The real die method includes two steps for measurement. One step is to measure a temperature sensitive parameter (TSP). In this step, a real current, e.g., 350 mA, is provided to the LED chip 100 by an operation current 102 and a voltage meter 104, and in another hand, a small current, e.g., 1 mA, is also provided to the LED chip 100 by a current source 108 and another voltage meter 110 in a temperature controlled environment for measuring a relationship between a chip temperature and an output voltage. Another step is to switch between the real current, i.e., 350 mA, and the small current, i.e., 1 mA, with a fast switch 106. Generally, a time of inputting the small current, should be less than 0.01% of a time of inputting the real current, for example shorter than 200 ms, or even shorten than 1 ms. Under this circumstance, a forward voltage under 1 mA is measured for the calculation of the chip temperature.
There are also some other conventional technologies for measuring thermal resistance. However, they are usually complicated, which often require the use of a metal-oxide semiconductor field effect transistor (MOSFET), or a rectifier diode, or even an OR logic circuit including at least two rectifier diodes. Otherwise, for an optical measurement, the substrate is just simply operated for obtaining the temperature control without capability to measure the thermal resistance. As such, those conventional approaches of measuring the thermal resistance are all restricted because the measurement of the TSP requires a stable testing condition which correspondingly requires a relative long time for getting the temperature balanced. Therefore, conventional approaches are not suitable for fast measurement of thermal resistances, optical characteristics of LEDs under different temperatures. Currently, these are the difficult issues in measuring the LED characteristics. As such, a critical concern in the art is to find out a solution and provide an apparatus and a method for measuring a characteristic and a chip temperature of an LED in a faster manner.